Manoeuvrability Final Edited

8/3/2019 Manoeuvrability Final Edited

1/12

NOVEL PROPULSION CONCEPTS OF MANEUVERABILITYNNOOVVEELL PPRROOPPUULLSSIIOONN CCOONNCCEEPPTTSS OOFF MMAANNEEUUVVEERRAABBIILLIITTYY

&&&

SHIP CONTROL - A PORT PERSPECTIVESSHHIIPP CCOONNTTRROOLL --AA PPOORRTT PPEERRSSPPEECCTTIIVVEE

SYNOPSIS

{This paper primarily seeks to address the design, manufacture and operational aspects of ship maneuverability, while doing sowe will also examine the various upcoming propulsion concepts, which have a direct bearing on the maneuverability &

operations. Main aim of the paper is to advocate the induction of the advancement and research regarding the various

propulsion ,ship control stabilization & steering equipment into modern shipbuilding practice so that ships of today and

tomorrow are more manageable, steerable & safer as well as more economic to operate when it is entering / exiting a port limit

or revisiting a channel or shallow patch. The subject of ship maneuverability or controllability is of continued importance to the

shipping community as maritime traffic adapts to suit the changing needs. The commercial restrains associated with current high

energy prices are shifting the balance between capital costs and running cost for marine transportation system..}

Presented By :SAPTARSHI BASU ,C/E,Fleet Management Ltd.(Author)SHISHIR DUTT,2/E,Exmar Shipmanagement India Pvt. Ltd.(Co-author)

INTRODUCTIONAmongst the many factors affecting the safety and the economics of ship handling , is the skill of the pilots

and masters, navigation systems and vessel traffic aids, as well as ships inherent maneuverability. As for the humanfriability is concerned a lot of work has been devoted to improve useful navigational aids to shipmasters in the form ofsophisticated collision avoidance systems, control and maneuvering data booklet available on the bridge . Additionallythe training of the crews using ship handling simulators have become mandatory.

An analysis has been made for large number of casualty reports, involving a total of 835 cases of CRG

(Collision ,Ramming & Grounding) damages, from USCG database using statistical methods. The causes of the

accidents show the following distributions:-

UNA VOIDABLE = 35%

HUMA N ERROR = 30%POOR MAN EUVERABIL ITY = 35%

________________________________________

TOTAL = 100%

____________________________________________

Referring to unavoidable casualties as to those whose causes were completely beyond operator control orvessel handling characteristics, further statistical studies on the influence of several controllability factors on collision,ramming and grounding damages (CRG) , suggest that quite a large number of CRG incidents could be potentiallyreduced by improving the maneuverability of the ships. Thus, claims by some quarters referring to the STCW-95convention that some 80%to 95% of the CRG damages are due to human error may not be entirely correct.

Th is top ic is inves t iga ted in de ta i ls under the fo l low ing head ing :-

I) MANEUVERABILITY WITH CONVENTIONAL PROPULSION STRATEGIES.

II) PERFORMANCE ANALYSIS AND COST FUNCTION.

III) SHIP CONTROLLABILITY REQUIREMENTS , ASSESSMENTS, VALIDATION AND

TRIALS.

IV) ALTERNATIVE PROPULSION CONCEPTS FOR BETTER DIRECTIONALCONTROLLABILITY.

V) CONCLUSIONS AND RECOMMENDATIONS.


2/12

(I)

MANEUVERABILITY WITH CONVENTIONAL PROPULSION STRATEGIES

When a ship approaches another solid underwater boundary (berth / Quay etc.) changes occur in localpressure field which can cause two ships close together to collide, or ship to hit a nearby bank / berth lying parallel to

its course. Shallow water also changes the local hydrodynamic pressure field around the hull leading to bodilysinkage of the ship known as squat. The total affect for ship traveling at speed in shallow water can be an increaseddraught by approx. 10%.Due to commercial constraints the modern ship building trends are towards larger ships withgreater freeboards with slow speed engines and simpler powering arrangements, frequently using ports whereoperational clearances in terms of available width in the locks ,depth of water under keel or room to turn are severelycurtailed. For large container ships operational schedules and commercial demands force them to berth near riverside or estuarial terminals, making the ship to operate in tidal conditions which tends to reduce the available safetymargins & the likelihood of grounding and collision are increased .The definition of ship controllability as given by,International Tank Towing Conference (ITTC) is Controllability is that quality of ship which determines theeffectiveness of the control in producing any desired change in a specific rate, in the attitude or position of a movingship.

(a) Steering control:-The most basic control required for the ship are control of direction/ heading of the ship, and of

its speed. The basic requirement for heading control is that the ships head is maintained within a given band ofdesired value which is known as steering error, which depends on the dynamic properties of ship, the effectivenessof steering arrangements, the disturbances present in form of wind and waves, and the human factor. The differencebetween the desired and actual course of the ship is called heading error. The helmsman or autopilot will act an thiserror and will alter the demand to rudder control mechanism which controls the rudder(s) to the desired angle after atime lag & within the bounds of error of the control system. Rudder at this angle acts on the slip stream of thepropeller(s) and creates a turning moment on the ship. The ship will turn under the combined effect of rudder, inertialand hydrodynamic forces, about a point which will usually be some distance forward of the mid-point of the ship. Insome ship,(such as VLCC), this pivot point, at which there is no component of sway velocity, is situated somedistance forward of the ship.The reason for siting rudders aft of the propellers at the stern of the ships is as follows :

1) It is able to exert a large lever arm about the pivotpoint and is able to position the ship such that the hydrodynamicforces assist in the turn.

2) The rudder at the stern of the ship is positioned so that the propellerslipstream augments the flow of water over the

rudder. The rudder forces are heavily dependent on the flow velocity of water across the rudder, the effectiveness ofthe rudder is enhanced by this positioning.

(b) Effects of disturbances:The most common disturbances acting on the ship effecting course keepingrequirements are winds and waves but other effects which effect the course control behavior will include:-

i) presence of seabed:- The ships behave differently in shallow water compared to the way they do in deep water.Turning ability is reduced and the diameter of turn is increased.

ii) presence of bank and other ships:-Ships tend to turn away from banks and there are complex interactionsbetween the two ships passing close to each other. The way in which the steering ability of the ship is affected by thepresence of wind depends on the shape of both the above water and underwater hull, and on the strength anddirection of the wind. The effect of wave will generally be to reduce the effectiveness of control mechanisms, and tomake steering within a given course margin more difficult.

(c) Speed control and fuel economy requirements:- Much effort is expended in ensuring that a ship isdesigned to operate as economically as possible. Fuel cost is one of most significant of ships operating costs .Mostof the merchant ships are designed to operate efficiently at a single speed, which may be the maximum/optimumspeed of the ship. Other ships, such as tugs, trawlers, OSVs etc have to be capable of operating efficiently over arange of speed and may not have very complex engine control arrangements to enable this to be achieved. This needfor range of speeds is frequently achieved by equipping the ship with more than one engine per shaft, so differentpowers and speeds can be achieved by using either, or sometimes both, of the available engines on each shaft.

Ships like offshore supply vessel during its operations may be required to achieve effectively a zero speed inpresence of wind and wave disturbances, while operating in its support role & required to maintain a precise station;for example during crane transfer operation. As soon as the transfer operation is complete, the vessel is likely totransit at a reasonable speed to a new station or shore. This dual role is achievable in number of ways. The normal

transit role is achievable by conventional means, while the positioning ability is achieved by directional thruster. Therequirement for precise control for engine speed is however much more onerous.The majority of merchant shipshave far simpler engine control requirements. With most cargo vessels being fitted with a single slow speed engineand designed for single speed, themain control requirement is simply that of maintaining a desired engine speed inthepresence of disturbances.


3/12

The basic cost cutting measures are crew reductions, fuel consumption reduction and simplified overallpropulsion system design. The more significant development towards great fuel economy have in recent years beenalmost universal adoption by deep sea trades of the slow speed diesel engines as a propulsion plant, with greater useof automated engine being able to both start and run on heavy residual fuels. While the propulsion arrangementconsists of a simply single large engine directly coupled to a single fixed pitch propeller. The disadvantage of suchsingle engine with single fixed pitch propeller installation from a ship controllability point of view are concerned withthe difficulties of slow speed operation, the problems of close quarter maneuvering and the inherent unreliability ofhaving only one main power source.

A slow speed diesel engine will rotate, at its slowest ,at about 22 rpm without stalling. This will correspond to insome ships, to a speed of 5 knots, approaching the berth. In order to go slower, the engine will have to stopped,which will result in a loss of directional control, because of the reduction of water flow across the rudder. Once theengine is stopped, it is to be started again using air from starting air bottles, which are being charged by aircompressors, hence there are a finite number of starts available to the master, depending on the charge level in theair bottles. In order to slow the ship, or reverse it, the engine has to be stopped and restarted in the reverse direction.The available air supply may not be able to start the engine in the reverse direction above a certain speed, so that theship cannot be always relied upon to achieve its full stopping performance.

(II)

PERFORMANCE ANALYSIS AND COST FUNCTIONSThe performance of a given system may be assessed by obtaining from a number of measurements, a set of

relevant data, and then preparing a combined figure which represents a meaningful measure of that performance. Anattempt is made to combine a number of desirable criteria into a single function, the maximization or minimization of

which will give a clear indication of the best solution.A similar process is used to assess the performance of a system under the action of a controller . A number of

relevant measurements of the performance of the system is made and a combination of these measures are used todefine a single quantity, which may then be used to produce the best or optimum performance of the system. Thismeasure is known as the cost functionorperformance criterion. A cost function is a single quantity which is usedto assess the performance of a system.A cost function is typically built up of a number of factors, each of which hasto be given a quantitative measure. In some cases the factors can be expressed in common units which may or maynot be expressed in money terms, while in other systems some form of weighing factor / criterion has to be preparedfor each other.

The cost functions for an auto pilot may be expressed in terms of two most important performance requirements,the course errorand the rudder activity. The cost function will be made up of both these component parts, so that itmay be written as :

CF = f (course error) + W x f(rudder angle)

The total cost function will be made up of sum of two components because the components will not be of equalimportance, both of them is weighed by different factors f for course error and W for rudder angle. The cost functionwill thus indicate by its value how well the autopilot is functioning . If both course error and rudder angle were zero forthe required period of time, the value of the cost function would be zero indicating perfect performance. In practicethis will not be feasible, because of the influence of disturbances and the ships directional instability. The aim,however, is to adjust the auto pilot to produce a minimum cost function.

Changing the value of the weighing factor will give a different emphasis to each of the two components of costfunctions, so that a minimum value of cost function, and hence an optimum performance, will be for a different type ofship behavior. Increasing W will give a greater emphasis to the degree of the rudder angle, and less to the amount ofcourse error. This type of performance would be suited to operations away from the confines of the port area, where itis more important to minimize the rudder activity than to steer a very precise course. For close quarter operations, it ismore important to steer accurately on course and large rudder angle are more acceptable, so that a small value ofWwould be appropriate. Thus varying W will change the type of performance, considering optimal performance.

(III)

SHIP CONTROLLABILITY REQUIREMENTS ,

ASSESSMENTS, VALIDATION AND TRIALS

The most significant factors of ships maneuvering which are most desirable from controllability point of view areas follows:-

1. Slow speed Maneuverability.

2. Adequate Backing power and straight line stopping ability.3. Short response time following rudder or engine demands.4. Adequate swing control with moderate rudder angle.

Any quantitative measure of controllability must therefore contain elements to assess each of the abovequalities.The design features from a Pilots point of view contributing to maneuverability include a large rudder, bow


4/12

and/or stern thruster, adequate ahead and astern power, reliable bridge equipment, a rate of turn indicator, goodbridge equipment layout, and finally a good visibility from bridge.

The correlation between a ships hull form and its maneuvering is not clearly defined as turning ability of the shipis governed by the balance of a large number of hydrodynamic forces.It should not be surprising that a small changein hull shape will cause large difference in behavior. A bluffer bow will cause hull hydrodynamic turning forces to beboth larger and situated further forward. Some bulk carrier with higher block coefficient enter turn quickly and haverelatively small turning circles diameter of under two ship lengths, but because of high hydrodynamic forces suchships may be slow to come out of a turn. This inability of ships with higher block coefficients to come out of a turn isreflected in a 10/10 zig-zag trial.

Conversely many ships with finer lines are stable directionally but cannot be said to be adequately controllable,as they do not turn readily enough in response to the helm. This type of hull shape will have smaller hullhydrodynamic turning forces acting further and so will not turn well . Hence turning circle diameter of six ship lengthsare not exceptional. Such ships , although being much more stable dynamically in turn than a bluffer bulk carriers ,still cannot necessarily be considered adequately controllable, as far as their directional behavior is concerned.

In case of turn taken by a dynamically unstable ship, when the rudder is thrown over to one side it deflects theslipstreams in the vicinity and cause the stern to move the ship initially, the bow will tend to continue on the samedirection so that the ship will be inclined at an angle to the original directions. Although the ship may have turnedthrough several degrees to the original course but it will not have moved significantly, in other words the ship is stillgoing in its original direction, although it has started to turn. This situation produces large hydrodynamic forcescaused due to deflection of the incoming water stream by the hull. The hydrodynamic forces act in the direction toincrease the angle of the ship to the streamline and hence to further increase the hull forces. It is thesehydrodynamic forces which largely turn the ship, the rudder producing little turning effect once the turn has beenestablished.

In a ship with smaller block coefficient, the initial action of the rudder is still to move the stream of the ship to theother side. Because of the shape of the hull the hydrodynamic forces are both smaller and centered further aft, sothat the turning moment is much less and the rudder will continue to assist in the turn. Rarely and unusually, someship designs are such that this is not the case and the ship might continue to turn in the original direction, until thespeed eventually falls so that the forces are to oppose the rudder, or power is removed to allow the ship to slow.

Where there is enough sea room, the use of turning maneuver to stop the ship is extremely effective with full formships, as the ship will adopt a high drift angle, presenting a large area to the flow of water. There are no national orinternational conventions or standards which require maneuvering or stopping ability of tank vessels to beconsidered in design process.

It can be inferred that assessing controllability is a matter of achieving balance between conflicting requirements.The problems facing a ship designer is that of ensuring adequate balance between these conflicting requirements.

(a) Effectors and Control surfaces:- Control of ship motions is exercised in two main ways , by imposingand including plane/surface to the stream line flow of water around the ships hull to provide mainly a force

perpendicular to the motion of the ship in the required direction , or by causing a surface to move in such a way soas to provide a reaction force driving the ship in the required direction. All such surfaces are known as effectors.

A range of rudder types has been developed which will enhance the turning ability of the ships, and the siting ofrudder in the slip streams of propellers will in general assist in turning response of the ships, as the rudder forces willbe increased by the faster wake they are in . Devices to enhance the lift forces of a rudder include:-

1. Flapped rudder, which operates by increasing the stall angle of the control surface, thus allowing effective ruddersideways at higher angles.

2. Rudder with a rotating cylinders in front of them which enable the increasing stream to adhere the control surface athigher rudder angles similarly increasing the effective lift of the rudder.

3 Rudders with special shapes designed to operate at large angles for example a shilling rudder which operates bydeflecting propeller slipstream to angle up to 90

0. The rudder is shaped so as to minimize staking effect at rudder

angles up to 750.

4 Flanking rudders :- In ships where rudder performance is of particular importance, multiple rudders might be

provided alongwith flanking rudders as provided in some towboats ,situated immediately forward of propellers andare used when going astern. The two steering rudders and flanking rudders are coupled together.

Alternatively methods of enhancing directional controllability of a ship include the use of auxiliary turning devicesuch as bow/stern thrusters, which provide a direct thrust athwart ships. These are usually situated at tunnels, thuslimiting their effectiveness to ship speeds below 4 knots, as hydrodynamic flow past the tunnel increases limiting

their effectiveness at higher speeds.On ships which have high need for good maneuverability such as tugs, offshore supply vessels and drill ships, a

range of auxiliary devices are used including:i) Azimuthing thrusters, where an auxiliary propeller is designed to be capable of rotating through 360

0. These

devices are limited in power and are frequently retractable.ii) Steerable nozzle surrounding the main propeller which operate by vectoring the streamline of the propeller . These

devices are limited by geometrical consideration to angles of about 45

0

.iii) Vertical axis propeller, such as Voith-Schneider system, in which vertical blades rotate in such a way so as toproduce a thrust which is variable in the direction through 360

0.

iv) A twin schilling rudder system, in which twin , independently operated rudders are situated immediately abaft thepropeller. Operating these rudders enables thrust to be produced in any direction from a single fixed pitchpropeller.Twin screwed ships will maneuver more readily if their rudders are sited abaft the screws, so that theyoperate in the propellers slipstream.


5/12

v) Triple screw and single rudder :-Some cross channel ferries which have to maneuver while going both ahead andastern have, sometimes triple screw arrangements and quite sophisticated steering arrangements for going ahead;triple screws being fitted with a single rudder abaft the center screw. This enables the ship to maneuver well byputting two outer engine astern and the center one ahead. This provides a net zero thrust, but allows for thedeflection of the center propeller slipstream, giving a strong directional thrust. For going astern a single low poweredpropeller and rudder are fitted at the bow of the ship giving additional directional control with engines astern.

(b) Assessing Controllability and ship trials:-It would be most desirable if controllability were able to beassessed before a ship is built, and some indications of methods of predicting it from ship plans are described. TheInternational Maritime Organization (IMO) has a sub committee on ship design and equipment which hasdeveloped draft guidelines for considering maneuvering performance in ship design. These guidelines define

specific maneuvering characteristics which quantify maneuverability and recommend estimation of thesecharacteristics during design stage as well as full scale tests to confirm the maneuvering performance estimation.These IMO guidelines are intended to be applied to ships greater than 10000 GRT, it was also felt by the IMO groupthat it is of importance to perform full scale test to validate design estimation. The most appropriate speed at whichthe test should be conducted is the maximum maneuvering speed rather that the full speed, to compare withnavigation in restricted waters, however deep water is preferred to shallow water .A lot of information for formulationof guidelines was received from European Research Community project COST-301, which includes investigations inseveral maneuvering areas, principally related to ship handling in confined water and vessel traffic systems. Therecommendation are given in the 1975 ITTC Maneuvering trial code as a guide to the performance of the trials.

(i) Assessing Controllability :- The usual approach is to device trials which are persistent to each of theperceived desirable qualities, to measure the results and to device a formula to relax the trial results to presumedperformance needs. At each step in this chain of events requires a decision to be made on what is suitable andrelevant. It is perhaps not surprising that there is a little uniformity in evaluating suitable measures of controllability.

Some agreement is however possible on what might constitute a suitable range of trials to determine some of theperformance characteristics of the ship.

(ii) Steerability cost function w.r.t. course keeping :- For a ship under automatic steering control,under a auto pilot, the most commonly used cost function for good steering is :

J = 1/T0T(2e +

2)dt where

e = Heading error , = rudder angle and = weighing factor depending on the ship type , loading condition etcThe heading error may be used to describe the effect of an increase of path length as well as increase of resistancea more generalized cost function will be of :

J = 1/T0T(a2e + b

2e + c

2)dtThis may be used to minimize fuel consumption, where

2e is concerned with increased path where

2eand

2terms are concerned with increased resistance the main draw back of these cost functions is that the sway velocity is

eliminated by taking it to be proportional to 2esince long period oscillations are considered. This is done because itis seldom possible to obtain an accurate measurement of sway velocity.The above cost function are usually associated with steering behavior in calm water, therefore1. the first order wave motions and measurement noise are , in many applications removed, by filtering2. different values of parameter of loss functions are chosen depending on weather conditions.

(iii) Ships trials purpose and conduct:- Ship trials can be inconvenient ,expensive and time consumingto carry out as they could require full availability of ship in good condition, under predetermined environmentalconditions often with extensive trial equipments and personnel embarked. Despite this, ship-trials are normallycarried out for one of the several purposes:-

1. As a contractual check at the end of a ship building contract, such trials are normally short in duration and limited tosmall number of contractual points.

2. First of class trials, when several ships of the same type and structural similarity (sister ships) are being built, it isworth while testing first ship to be built more extensively, as a check on overall effectiveness of design on the hulland machinery.

3. As a check on maneuverability, same will be detailed later.4. To provide data for ship maneuvering mathematical models. These trials, as they need to examine the ship behavior

in all maneuvering regimes, will be very extensive in scope.The desirable features of particular ship trials are :-

1. the starting conditions and tests inputs are easily reproducible.2. It should be conducted and recorded with a minimum of added equipment.3. The trial should bear a close relationship to actual ship operating conditions.

The USCG has proposed the development of new regulations concerning the maneuvering and stoppingcharacteristics of the US ocean going tanker, passenger vessels, cargo and great lake bulk carriers. Theaccomplishments of these standards will also furnish the required information to be posted in wheelhouse. An

improvement in the inherent maneuverability of the ships can significantly reduce the number of CRG accidents,coupled with the risk of pollution. Maneuvering in restricted waters, course keeping, initial steady turning ability, yawchecking and stopping ability are the most widely recognized maneuvering abilities of the ship. Further more eighttypes of controllability particulars are recognized for CRG prevention as follows:-

1. The ability to maintain control of vessel, after loosing one steering / propulsion unit.2. The ability to slow down while maintaining control of vessel after loosing one steering / propulsion unit.


6/12

3. The ability to slow down while maintenance of steerage in Y knots of wind speed and Z knots of current speed.4. The ability to maintain the heading of the ship as it is affected by ship / bank / bottom suction / sheer at X knots of

ship speed in Y knots of wind speed and Z knots of current speed.5. The ability to control the heading of the vessel at X knots of ship speed in Y knots of wind speed and Z knots of

current speed.6. The ability to turn the vessel more sharply with X knots of ship speed in Y knots of wind speed and Z knots of current

speed.7. The ability of the master to obtain additional power in Y knots of wind speed and Z knots of current speed with W

number of tugs already assisting.8. The ability to control the heading of the vessel backing at X knots of ship speed in Y knots of wind speed and Z knots

of current speed9. The ability to stop the vessel from X knots of ship speed in Y knots of wind speed and Z knots of current speed in W

minutes.The requirements 4 ,5 and 8 refer to course keeping , turning and stopping abilities while introducing external

disturbance and 7 refers to astern course keeping. The remainder are typical needs of controllability in restrictedwaters and a source of potential casual risk.

A data bank of maneuvering trial results of some 600 ships have been created. The data corresponding todifferent maneuvers have been analyzed statistically and graduated in terms of indices. With the objective offacilitating the task of the regulatory bodies concerning the establishment of maneuvering standards a set ofrepresentative indices have been proposed as follows.:-

1) Turning and course keeping:-A) tactical diameter / length at 35 rudder angle.B) Overshoot angle from 20/20 zig-zag maneuver.C) K-t relationship from 20/20 zigzag maneuver.

2) Stopping:- Head reach / length from a crash stop maneuver.

3) Crash-stop maneuver:- Head reach / length from a crash-stop maneuver.

4) Ability to operate at moderate speed:- Continuous operation at a speed between four to six knots for all ship loadconditions.

5) Operation under severe environmental conditions:- Satisfactory operation for any loading at the respectivesevere wind condition. As a consequence of this characteristic , the maximum above water surface would be limited,and will be function of the submerged area , as well as ship and wind speeds.

6) Course change test :- Though not included in the ITTC maneuvering code this gives invaluable data regarding thesteerability of the ship and its fundamental design of the auto pilot.

For any trials it is necessary for the ships position to be fixed relative to either a shore or to the water at intervalsof every few seconds. This may be done using shore based tracking equipments viz geodimeter. It is also possible tomeasure ship position using ship-borne set of equipment incorporating a inbuilt navigation package (Gyro pilot)which will give the acceleration of the ship.

To assess maneuverability/controllability ,one or more trials will be necessary for each of the desired qualitiesreferred above. The trials most commonly associated with measuring controllability are the Crash- stop, the turningtest or circle maneuver and the zigzag or Z maneuver carried out at full speed and full maneuver.

The crash stop is maneuver to test the vessels stopping ability. The ship proceeds at a straight course at ameaningful speed for the maneuver concerned, and the engine is put to full astern. The distance traveled before theship stops is measured along with the time taken to stop.

The circle maneuver is of particular interest in that it gives information about transit and steady statemaneuverability of the ship. With ship running on steady course & speed ,the rudder is pulled over an appropriatepredetermined angle and held there. The ship then starts to turn and eventually reach turning condition. The

measure of interest will include the advance, transfer and turning / tactical diameter, which gives information of theships turning ability. If the helm is put to mid-ships after the ship has been reduced to a steady turn a simpleassessment can be made of its course stability. This maneuver is called pull out maneuver. Unstable ships willcontinue turning while stable ships will return to an approximately straight path. As far as controllability is concerneda degree of instability can be advantageous, as turning and checking ability can be faster than with a similar size ofship with a directionally stable hull.

The zigzag maneuver is perhaps closest to the actual ship operations. The ship again starts straight line at aconstant speed and the rudder placed over by specified amount, typically 5 ,10 or 20, depending on the size andmaneuverability of the ship. After the ship has turned through a determined number of degrees,called the checkangle; the helm is put over the other way by an amount equal to the rudder angle and the ship will then turn in theopposite direction, until the check angle is reduced on the other side of the initial course, when the rudder is againreversed.

Measures of interest are the overshoot angles after the helm has been reversed and the swept path of the ship.The additional swept path after the helm has been reversed may also be of interest.

For controllability assessments, slow speed maneuver with moderate helm angles are more likely to be of interestthan full speed maneuver with large helm angles. The reduced speed trial is designed to assess the ship ability tosteer at 4 and 6 knots.

(iv)Predicting Controllability:- It would be preferable to be able to predict the controllability of the hull-effectors combination at a very early stage in the design of the ship. To find out at a very early stage is much better


7/12

than to find that the ship has undesirable control characteristics at trial time or even at service, which cannot beconsidered as sound design practice. It can be seen though, that the controllability of ship is made up of a number ofaspects some of which such as the swing control, depend on the interrelationship of both the hull and effectors.Captive model trials are unlikely to give more than a indication of the controllability , as they rarely are realisticrepresentative of the hull and the rudder interaction in maneuvering conditions. Some progress is being made withthe ability of researchers to model the maneuvering ability of the ships before they are built from a combination offree running model tests and mathematical models.

(v) Improving controllability:-If a ship is found to have an adequate controllability a number of measures canbe taken to improve the controllability situations in some cases. Where the rudder performance is inadequate ascould be shown in the excessive first over shoot angle in a zigzag trial, a rudder with greater lift can be retrofitted.

Similarly stopping ability and slow speed maneuvering ability maybe able to be improved by fitting a controllablepitch propeller. In some case, however, the basic hull form is at fault, leading to excessive overshoot and virtuallyuncontrollable behavior. In these cases some degree of palliative action may be taken by increasing the size of theskeg after, increasing the size of the rudder or by fitting additional control surfaces. These attempts to improvecontrollability on an otherwise uncontrollable hull are not in themselves a satisfactory solution, but merely a way ofmaking the best of poor design. A hull should be inherently controllable, without relying on imposed devices to makeit so. Of particular interest is the precise shape of the forward part of the hull, as the hydrodynamic lift and dragforces are to a large extent generated at the fore body of the hull. If the fore part is too bluff, generating an excessiveamount of lift at the forward extremity of the hull, it could be that hull will become totally uncontrollable inYAW withnormal control surface.

(vi)Controllability as a Port-ship problem:- The controllability of a ship doesnt only affect the ship itself.If a ship has poor level of controllability, it is likely to require greater sea room for maneuver or will be operatingcloser to margin of safety in port. For example, if a high sided container ship with a single speed diesel engine with

fixed pith propeller may have too high Speed Over Ground (SOG) in a strong favoring current, due to minimumstalling speed requirement of diesel engine, besides it will also lack directional controllability at very low speed, whilecrossing very narrow passages or height restriction such as bridge pier, especially if a transverse thrust by a strongwind or under current is imposed by such area. Under these conditions safety conditions are minimal. Techniquesare developed to assess the suitability of such ships to transit a port area safely in the range of environmentalconditions. The simulation requires a mathematical model of the ship under consideration, with its controllabilityadequately modeled. This can be achieved by using a range of techniques, including carrying out special ship trials,performing free running model trials,or by modeling the ship directly from the knowledge of its lines, plans andeffectors design. A number of such models have been produced which have been tested against actual ship results,and a measure of confidence is able to be built up in the techniques. The use of ship simulators for developingsuitability of a port for particular ship operations is now becoming a routine part of port design process.

(IV)ALTERNATIVE PROPULSION CONCEPTS

FOR BETTER DIRECTIONAL CONTROLLABILITY

The most popular arrangement for the commercial ships is a large diameter as slow turning as possible ,single fixedpitch main propeller in stern coupled with a large slow speed cross-head type diesel engine and fixed speed sidethrusters fitted at the bow, this is for reason of simplicity, efficiency and the need to minimize initial costs, with abalanced type of spade rudder aft.

On this type of arrangement a maximum propeller efficiency ofabout 70% can be obtained, the 30% loss can be split into 3

parts:- About 10% due to the loss in momentum About 10% due to loss in friction About 10% due to rotation in the propeller race.

There are other propulsion arrangements other than thesimple single screw propellers which may result in higherpropulsion efficiency of the ship. All these devices haveimproved maneuverability, flexibility and factor of safety andclaim to gain in efficiency. Some of the units are complicated indesign and expensive to manufacture, which may haveprevented their wide spread acceptance in past. Same is thecase with thrust augmentation devices are hydro-dynamically

based energy saving devices which react with the final stage ofgrowth of boundary layer around the stern of the ship or the hull wake field or propeller slip stream attempting torecover energy which would be otherwise be lost.

1) Multiple Screw arrangements:- The choice of the number of screws for a merchant vessels depends onmany factors, such as amount of power to be transmitted, draught of the ship, position , height and type of the


8/12

engine, the margin of safety available in certain cases to ensure that the failure of one engine will not disable thevessel, economy of the running costs, initial expenses and efficiency of propulsion. The use of twin screw propulsionis an adaptation of utilizing the largest possible diameter of the propeller, but in case where the diameter is limited bydraught of hull multiple propeller allow greater mass of water to be utilized, than a single screw alternative. In twinscrew the benefit form the energy from the wake is lost due to the off-centre position of the propellers but this can beobtained with triple screw where the centre-screw absorbs most of the power. The loss in efficiency is compensatedby increased maneuverability as each propeller can be rotated ahead and astern independently. The propulsivecoefficient of a triple screw is slightly better than twin screw, which may be ascribed to the better efficiency of thecentre screw working in the frictional wake of the ship and propulsive coefficient is increased by the rudder behindthe centre screw and the resistance is less than the corresponding twin screw owing to the smaller dimension of thebossing.

One advantage of a triple screw arrangement is that it makes it possible the stopping of one of the three enginesduring the voyage for the repair without any undue loss to the propulsive efficiency. If the engines have clutchingmechanisms, the engine and the propeller can be disconnected so that the latter can be freely turned in the waterand need only overcome the frictional loss at the shaft bearings.In multiple screw ships the direction of turning of the propeller is such that the blade tips turn outwards when in upperpositions. With outward turning screw both ahead and astern motion, the resultant is shifted outwards with respect tothe centre of the screw shaft owing to the unequal distribution of the intake velocities at the screws. A twin Screwship with outward turning propeller can therefore be steered better by means of the propellers than they if wereinward turning, as in latter case, the point of application of the thrust is moved inwards with respect to the centre of

the screw-shaft so that the turning moment becomes smaller.There are other types of two propeller arrangements which are non-conventional and have their own advantage:-

Tr ip le Screw Arrangement

Tandem Propellers

Overlapping or interlocking propeller,

Two propeller placed vertically above one another

Contra-rotating propeller arrangement

The propellers on the same shaft and turning on the same direction are called Tandem propeller .As aftpropeller works in the same race of the forward ,forward one requires a higher pitch to give same power

absorption.With both the propeller rotating in the same direction,the rotational energy in the race is augmented by the working ofthe after-one.

In the interlocking or overlapping propeller arrangementthe propeller disc overlap each other, the two propellers of thenormal twin screw can be moved aft to a longitudinal position ofnormal single screw propeller and inwards until the distancebetween the shafts is less than the diameter of the propellers ,which in this ways interlocks in the centerline zone. This

combines the efficiency of the twin screw with the efficiency of

the single screw system propeller ,leading to lowappendage resistance and high hull efficiency . Anarrangement due to both screw turning in the samedirection may be better owing to better energy recover,the optimum distance between the shafts, is 60~80%of the propeller diameter. The separation in thelongitudinal direction has only effect on the efficiencyand is primarily of importance in averting the danger of vibration occurring. The U-shaped transverse section used insingle-screw vessels, particularly favor this propeller arrangements unlike the V-form usually found on the on twinscrew installation. In this propeller shafts will have to be connected with the gear for being driven by the single

engine. The required power is 10% lower than in the single screw arrangement. The arrangement has the followingfeatures:-

The total jet area is smaller ,that is, a reduction in ideal efficiency.

Propellers operate in an area of the concentrated wake which increases the hull efficiency.

There may be some effects from mutual interaction.

A rearwards converging shaft arrangement gives better propeller support.


9/12

Recovery of rotational energy with both propeller turning in the same direction.

The resistance of open-shaft bracket and shafts placed obliquely in the flow is lower than the conventional singlescrew arrangement.The decrease in jet area and the possibility of utilizing the concentrated wake mutually influence efficiency. Theoverall propulsion efficiency attained is higher than conventional design. A reduction in resistance of struts andshafts brings about a decrease in power required, corresponding to one third of this part of resistance.Interaction effects can cause vibration and cavitation, both of these can be overcome by the setting of the blades . Itis advisable to have different number of blades on port and starboard propellers. The following are the variants:-

1. Direction of rotation of the propellers.2. Distance between the shafts.

3. Clearance in the longitudinal direction4. Stern shape.5. Ship block-coefficient.

2. Rudder Propeller :- It is an auxiliary propulsion and steering device. The rudder mounted propeller have thefollowing advantages:-

1. Good control, especially when starting from rest.2. Unlike lateral thrust propellers, control effect improves with speed.3. If necessary, a lift mechanism can adjust the unit to ships draught .4. To reverse the vessel the propeller is swiveled 180 degrees for astern action.

The disadvantages are:-1. propeller has to be fitted to the support mechanism, which increases resistance.2. Complicated Z- Gearing.

3. Limited power in current design.4. Influence of the jet with hull or the adverse effect caused by interaction with main propeller race.

3) Grim wheel:-It is a vane wheel with larger diameter than propeller ,freely rotating like a turbine mounted on anextension to the propeller shaft aft of the main propellers.The grim wheel has morenumber of blades than the usual propeller and is about 20% larger in diameter. Thevanes of the grim wheel are so designed that the inner section acts as water turbineto extract from the propeller slip stream a large amount of energy which wouldotherwise be lost. The recovered energy is converted directly into additional anduseful thrust in the aft part of the vanes which acts as a propeller. This function ofmaking use of the active propeller spin and the jet energy results in savings either inthe form of increase in thrust and ship speed or reduction in the horsepower inputrequired for a given speed. The improvement in propulsive efficiency ranges from 5%

to 15% depending on the type of the vessel.The unit rotates on low friction roller bearings on a steel stub shaft flanged to thepropeller boss and the radial lip seal prevent water ingress to the bearings. it can beused with either fixed or CP propeller .Compared with a conventional propellersystem the addition of a grim wheel brings a following advantage:-

1. Propulsion efficiency increase upto 15%2. The main propeller can be operated at a higher rpm favorably affecting weight and cost of the prime movers.3. Grim wheel can be as large in diameter as possible since the large number of blades and low speed of the wheel

allows small vertical clearances with the hull to be acceptable.4. There is less resistance from a rudder fitted behind the grim wheel, this is shown in the relative rotative efficiency.5. Ships fitted with this device have better stopping capabilities.

4) Shrouded/ducted propellers( Kort Nozzles) :- These arrangements are designed to increase the flexibilityof the engine /propeller combination but dont alter the propeller as such. The device is based on momentumconsideration and under a certain condition cause a large increase in maneuverability and performance. Forexample, Astern maneuverability with a Kort nozzle is better at high thrust level. Efficiency improvement of upto 20%compared with conventional propeller efficiency seaways and increase of bollard pull of upto 30% is achieved. TheKort nozzle named after the inventor, is an annular forward extending duct of aerofoil section fitted around thepropeller. It may be fixed or steer able type with nozzle replacing the rudder and supported by the rudder stock andpossible lower pintle at the keel.

The cross section shaped as hydrofoil, comes in three types of shapes, viz accelerating nozzle, neutral anddecelerating nozzle. The ducted propeller with accelerating flow type is now commonly used where the screw isheavily loaded. The nozzle itself produces a positive thrust. It will be noted that the parameter for the amount of gainis a measure of slip and not much of speed. This means that the shrouded propeller is much more advantageous tohigh slip vessels such as the V.L.C.C.s bulk carriers, tugs and trawlers etc.

In a fixed type of nozzle the propeller operates with a small clearance gap between the blade tips and the nozzleinternal wall at the narrowest point. Nozzle Dihedral angleis the angle between the nozzle axis and the line joiningthe leading and the trailing edges. If the dihedral angle is increased then the outlet section will be severely narrowed,while if it is too small the flow diffuser angle will become too separated and eddying. It is this angle which will causethe water to be accelerated to the propeller suction field, generates the additional thrust on the nozzle body itself,which is transmitted to the hull through the fixtures. These are predominantly fitted with Kaplan propellers which havethe best efficiency in these types.


10/12

5. Azimuthing thruster:- It is a rudderless Z-drive propulsion system where a

propeller is mounted on a slew-able shaft supported by the ships structure. Thenormal azimuthing mechanism is a worm wheel drive mounted on the head toallow the complete unit to be turned through 360 so that the propeller thrustcan be directed anywhere in the horizontal plane. The combination of steeringand thrust executed by one mechanical device which gives the following majoradvantages:-

1. Most effective application of thrust as a whole unit is turned to direct propellerthrust where it is wanted , rather than relying on deflection by rudder.

2. Very precise & effective steering of vessel, through thrust direction.3. Ease of vessel control through single lever system or microprocessor controllers

linking more than one unit.4. Much simpler machinery installation with no shaft bearing housing to be bored,

giving flexibility in stern design of vessel.5. Great flexibility in choice of propulsion units such as direct drive electric motor,

hydraulic motors, etc. If a constant speed C.P. propeller is used , use can bemade of engine or transmission system.

6. Engine room length can be remarkably shortened meaning the payload sectionof the vessel can be increased by 5% to 10%.

6. Controllable pitch propellers( C.P.P) :- Controllable pitch propellers haveblades separately mounted on the hubs, each on an its own movable axis ;the

blades can be turned about these axes leading to change of pitch of the blades, andeven reversal of the pitch while the propeller is running by means of an internalmechanism in the hub. The forces necessary to turn the blades are relatively largeand for this reason the area of the blades tends to be lower than normal. The bossdiameter ratio is 50% greater than the fixed pitch propellers under the sameoperating condition. These propeller have also been known to cause CartwheelEffect ,at zero/low pitch at full rpm, which causes the stern to swing in the direction ofthe propeller at the top of the circle. The main advantages of the propeller despite theinitial first costs are as follows:-

1. Rapid reversal of thrust from ahead to astern and practically unlimited number ofreversing not dependent on the air bottle pressure.

2. Full power in astern thrust, compared to steam turbine direct couple diesel engine where engine parameter doesntensure optimum performance during astern maneuvering due to timing consideration.

3. Optimum utilization of power at partial loading when the engine rpm is reduced .4.The engine overloading can be avoided by automatic pitch controlsystem to reduce pitch, if the engine overloading is maximum and thepermitted cylinder pressures are likely to be exceeded.5. Constant speed operation means operation in the optimumspeed range leading to fuel costs savings and avoidance of thebarred speed range during maneuvering.6. Possible improvement in the propulsive efficiency, simpler engineplant {e.g. gas turbine}, greater maneuverability, less wear and tear.7. Spare propeller blades are cheaper than a complete solidpropeller of fixed pitch type. In a standardized fleet with one type ofc.p.p. spare parts costs are reduced.8. The ability to drive shaft generators and pumps from the main

engine results in direct savings in fuel costs, particularly if aux engineoperates on diesel fuels.9. Greater maneuverability means increase in savings from harbourcosts if tugs are not used.

7) Contra-rotating Propellers and podded C.R.P :- The concept of contra-rotating propellers was originatedby Erricsion on 1839. It is an arrangement with two propeller on the same shaft or shaftcenterline rotating in the opposite directions. The propulsion system operates largelywithout exit rotation losses which is 8 to 10% for conventional propeller. The aftpropeller disc diameter is somewhat less than of forward one to prevent cavitation dueto the tip vortex of the fore propeller hitting the aft propeller and for optimum propellerefficiency. The diameter of the aft propeller is 10 ~20% less than the fore propeller and

may have different no. of blades with different pitch. It is about 16~20% more efficientcompared with large propellers of large vessels such as VLCCs. Additionally the inputpower is shared by two propellers instead of one, this reduces propulsive efficiencyand thus delays the onset of cavitation.

This was predominantly used by torpedoes for propulsion due to the natural torque compensation giving muchbetter directional stability. CRP have recently been applied to, large merchant ships. The mechanical complicationinvolved in turning two propellers coaxially in opposite direction requiring the boring of outer shaft for mounting the


11/12

inner shaft with bearings with outer sealing arrangement and the installation of the star type planetary gear,necessary for two driven shafts to turn in opposite direction, when one main engine is used, along with additionalcosts involved was not considered justified and till now has inhibited the use of CRP.

This problem was solved by using a much simpler methods by podded CRP concept where a contra-rotatingpropeller on an electric pod or Rotatable Z-drive thruster is located directly behind the main propeller in the centerlineskeg. The two propellers are arranged as close as possible to each other hence the rudder blade section of thethruster is behind the propeller which can act as the pulling thruster. The pod propeller is of fixed pitch type, while themain propeller is of controllable pitch type.The podded configuration offers a better hydrodynamic efficiencycompared to the conventional vessel with a single twin screw due to the following reasons:-

1. The resistance of a single skeg hull form with a single pod is lower than that of a twin screw hull with two shaft lines.2. The aft propeller takes advantage of the rotational energy left in the slipstream of the forward propeller.3. The skeg offers a more favorable wake than a shaftline, resulting in better hull efficiency H.

The podded CRPs offer excellent maneuvering due to the azimuthing pods giving much higher turning momentdue to the directional thrust vectoring. The optimization of power split between main shaft and the pod is importantas electrical transmission per kilowatt is more expensive than mechanical transmission and the z-gearing has aceiling power rating due to current design limitation. The solution is that a 25/75 power split between pod and mainpropeller is the best option from the total efficiency point of view. The economical factors also favor a small pod. Thecavitation performance of the pods as measured in towing tanks is on a good level for the CRP concept and betterthan conventional system.

8) Waterjet and pumpjet :- A water-jet is basically a marine propeller in water fed through an inlet duct at thebottom of the vessel, to an axial flow water pump which adds energy before expelling the water through the nozzle ata much higher velocity than the incoming stream. The resultant change in momentum provides the thrust to the

vessel; and the jet can be directed either side by a movable nozzle and/or scoop to provide steering and reversingcontrol. Waterjets are typically driven by diesel engines (medium or high speed) or even gas turbine and are popularfor high speed craft and vessels required to operate in shallow water. A major advantage for the waterjet is that for agiven power loading a higher static thrust can be obtained compared to an equivalent conventional or shroudedpropeller. This is because for the low speed application the pump can be operated close to the point of optimumefficiency. Another feature is the ability to absorb full driving engine power at all water speeds without cavitation.Compared with conventional propeller this results in superior accelerating characteristics as well as directionalcontrol as maximum thrust is immediately available. A typical design of waterjet has an axial flow impeller bladeswith very wide chord running close to the pump casing with minimum tip losses and stator blades to strengthen thewater flow down stream of the impeller to avoid rotational losses.

To overcome the problem of debris a grille is normally fitted at the intake .For steering, the outlet nozzle pivotsabout an axis set at 45 to the vertical using hydraulic rams ,are provided. When the nozzle is swung either side ofthe straight ahead position the whole jet stream is swung with a minimum loss for positive steering action. Selection

of ahead through neutral and astern is obtained by progressive lowering of scoop or bucket which rotates about ahorizontal pivot mounted on the outlet nozzle.Jet units with power upto 15,000 Hp/Units are used in militaryamphibious vehicles, workboats and high speed catamarans.

A variation of the waterjet is the PUMPJET developed by Schottle. This is basically a centrifugal type pump inwhich impeller axis is mounted and works in a volute type casing. The suction port is mounted against the flat bottomof the boat and the water is energized in the pump and expelled through a nozzle back into the water down at anangle of 15, at the bottom of the craft. The complete volute casing can be rotated through 360.

9)Vertical axis or Cycloidal propellers:- The vertical axis propeller has a number of blades of aerofoilsection connected perpendicularly to a disk whose axis of rotation is vertical. The blades of the propeller are fullyimmersed and all other parts such as rotor are housedinside the hull. The bottom of the rotor is flushed with theshell plating of the ship. During the rotation of the propeller,

the blades perform an oscillating motion , which isperformed by mechanical means inside the rotor. Thismotion is regulated according to the two components, one ofwhich takes care of propulsion other takes care of steering.The oscillating motion blades of a vertical axis propeller isgoverned by a system of rods and hingesThere are two types of vertical axis propellers:-

i. Kristen-Boeing :-It has blades which are so interlocked bygear so that each of the gear is constrained to make half arevolution about its axis for each revolution about their wholepropeller. The orbit will be a cycloid.

ii. Voith-Schneider:- Its blades complete full revolution aboutown axis for each revolution of disk. The orbit will beepicycloid.The normal arrangement has a bevel gear drive of the vertically mounted propeller shaft and onlyconnection between the engine and the propeller is a torque converter which absorb shock loads and rapid pitchchanges and provides no load on starting of the engine.

Some units have a unit at the stern and one at the ford .With a two propeller arrangement a strictly a lateralmovement can be obtained by directing the thrust athwart ships perpendicular to the lateral plane. The effect of thepropeller is such that by adjusting the attitude of the blades, the power absorption can be varied from the zero to the


12/12

full power i.e. it is a controllable pitch propeller. Also by further variation inblade attitude the thrust can be directed to any direction through 360. As faras the propeller operation is concerned, there is no difference in the aheador astern command, or sideways. Thus, these propellers are used for normalsteering and no rudder of any kind is required. This propulsion gives anabnormally high degree of maneuverability, but the thrust of the propeller islower than with a shrouded propeller, thus larger powers are required toobtain the pull required.

The propeller itself and itself and its mechanisms are complicated inconstruction and is commensurately costly both to install and maintain. Thehull must be especially constructed to accept these propellers. Thesepropellers may not be used in high speed vehicles.

(V)CONCLUSIONS AND RECOMMENDATIONS

In so far we analyzed the concept of maneuverability, how it is assessed and verified by model testing. We are alsoaware of how maneuverability is affected right from the design stage and analyzed all the parameters effecting the

maneuverability and propulsion performance. Lastly we have analyzed all the major alternatives propulsion modeswhich are mostly advantageous in terms of propeller efficiency, increased maneuverability or both. We have alsoseen in details the interplay between the various major components of a propulsion and maneuvering system.

At present time, various regulatory bodies are considering formulation of rules concerning ship maneuverability, inso far as it influences safe navigation of ships. Proposals have been made for a number of criterion to be satisfiedduring design of a new ship, which would be subsequently be verified during full scale trials. In view of the proposeddesign requirements of the regulatory bodies, the ability to specify the hydrodynamic and aerodynamiccoefficients either theoretically or semi-empirically would be of immense help. This could be achieved by continuingto examine the means of correlating the captive model data, involving the co-operative efforts amongst variousbodies.The problem of ship maneuvering in extreme condition persists as many aspects have to be satisfactorilyanalyzed and examined, the maneuvering in transient and non-uniform condition shall receive special attention.

Classification Societies are largely silent on aspects of maneuverability and controllability. While most societieshave extensive rules about the construction of rudder, the guidance to their effectiveness is largely missing. It will benoted that while the need for adequate maneuverability is stressed, no attempt is made to give any quantitative andqualitative expressions to this quality. This is in marked contrast to the detailed specifications used for all structuralaspects of the ship design. Even by regulatory bodies no guidance is given on maneuvering qualities expected ofships.

Ship owners might be interested in the controllability of their ships, in an expectation that more controllable theirships are, safer it would be to operate hence less frequency of collision or groundings. Ship owners are mostlyinterested in transporting their cargoes at a minimum costs. Fuel saving measures will frequently be incorporatedinto a design, as the costs of these measures can readily be set against savings. The costs of additional measures toenhance controllability of the ships may similarly be readily identified at the design stage, but the consequent savingsare not readily apparent to the ship owners. Also there is no widespread accepted quantitative cost functions, whichcan be used to measure the gain in controllability of a ship, so it is difficult for the ship-owner to be well advised onthe potential improvements to be gained from enhanced controllability.

Insurers might be expected to have direct interest in controllability of ship on grounds that a more controllableship vessel could be expected to have fewer losses than one with poor controllability. The response most frequentlyfound from the insurers, however, is that design of the ship is for the ship-owners and the classification societies andnot the insurers.

Port Authorities should be the most interested in the ability of the ship to maneuver safely into their port, and it ishere that there has been maximum and most direct interests shown by some authorities. The desirable feedbackpath from the port authorities through the ship-owner to the naval architect,which could lead to safe and morecontrollable ship designs, is largely absent.The current situation on controllability appears to be:-

I. there are no agreed standards on ship controllability.II. Without such standards, no classification society or government department is in a position to issue codes or

regulation governing controllability of the ships.III. Without enforceable codes ship-owners cannot quantify the naval architects the degree of controllability they require

in their ships.IV. While port authorities can recognize a poorly controllability and design their ports accordingly to take account of itthere is no feedback to ship designers,

The result of this situation is that ships continue to be designed and operated with unnecessarily poorcontrollability. The design of theses ships will be governed to a large extent by consideration of financial costs, bothinitial and running.

Manoeuvrability Final Edited

Documents

Transcript of Manoeuvrability Final Edited